Membrane for fuel cells

ABSTRACT

The invention relates to a membrane, in particular, a membrane for use in a methanol fuel cell. The inventive membrane comprises complexing agents for cations and, therefore, functions like an anion exchanger. In a particular embodiment, the membrane comprises complexing agents selected from the group of crown ethers, cryptates, or of cryptate-like compounds based on carbon cyclic compounds or silicon compounds.

[0001] The invention relates to a membrane, especially a membrane foruse in a fuel cell, as well as to a fuel cell containing such amembrane.

[0002] A fuel cell comprises a cathode, an anode and an electrolytedisposed between the anode and cathode. An anode chamber adjoins theanode and a cathode chamber adjoins the cathode.

[0003] In a methanol fuel cell, methanol is fed as working gas to theanode chamber. On the cathode side, oxygen or air is supplied asoxidizing agent. By means of a catalyst, hydrogen ions are formed fromthe methanol at the anode. The hydrogen ions pass through theelectrolyte, which typically comprises a membrane, and on the cathodeside combine with the oxygen ions formed from the oxidizing agent toproduce water. In the process, electrons are liberated and electricalenergy is generated.

[0004] From German Patent 196 46 487 C2 it follows that the electrolytesof known fuel cells allow not only the hydrogen ions (protons) to pass,water in the form of hydrate shells around the protons and even some ofthe fuel (CH₃OH) can also pass. This means on the one hand that too muchwater is transported from the anode to the cathode, thus adverselyaffecting the thermal balance, while on the other hand methanol is alsoadditionally transported, thus leading in general to development of amixed potential at the cathode and thus to a perceptible loss ofefficiency.

[0005] An electrolyte used typically in the direct methanol fuel cell(DMFC) is the cation-exchange membrane Nafion®. With this membrane also,the proton transport from anode to cathode is again accompanied bymigration of methanol and water through the membrane. This results inpotential changes on the cathode side and in destruction of thecatalyst. Furthermore, the methanol loss causes reduction of theefficiency of the fuel cell.

[0006] One alternative would be to use a separating layer ofanion-exchange material. In this way the problem of methanol transportthrough the membrane could be greatly reduced in principle, albeit atthe cost of higher ohmic resistance and higher material costs.Heretofore no suitable materials have been found for this purpose.

[0007] In conventional anion-exchange materials, the substituted groupis typically an —NR₃ ⁺group (quaternary nitrogen group with R=—H or-alkyl). It is known that this group itself is not very stable, and sothe stability and physical properties of such anion-exchange materialscan be maintained for only a short time compared with correspondingcation-exchange materials. This is particularly true in the presence ofstrong bases and higher temperatures, as follows from “Ion PermeableMembranes”, by T. A. Davis, J. D. Genders and D. Pletcher, AlresfordPress Ltd., 1997, p. 20.

[0008] The object of the invention is to provide a thermally stablemembrane that does not suffer from the foregoing disadvantages. Anotherobject of the invention is to provide a fuel cell in which thedestruction of the cathode by methanol as well as the change ofpotential at the electrodes is largely prevented.

[0009] The object is achieved by a membrane having the features of claim1 and by a fuel cell having the features of the alternative independentclaim. Advantageous embodiments are specified in the respectivedependent claims.

[0010] The inventive membrane according to claim 1 containscation-complexing agents. For example, a typical membrane of this typecould comprise individual polymer chains, whose side chains contain thecomplexing agents. Such a membrane can be synthesized, for example, bylinking crown ethers via phthalocyanine units, as described hereinafter.Benzo-18-crown-6 is brominated in 4,5 position and reacted with acyanide to obtain the substituted phthalodinitrile. This step isfollowed by a phthalocyanine synthesis.

[0011] A further example of an inventive membrane is obtained by theaforesaid synthesis scheme by using dibenzo-18-crown-6 instead ofbenzo-18-crown-6.

[0012] The inventive membrane permits orderly ionic transport of anions,especially O²⁻ ions or hydroxyl ions through the membrane, butadvantageously prevents cation transport, and especially the transportof larger molecules such as methanol.

[0013] Furthermore, the inventive membrane is thermally stable andinsensitive to bases. It is advantageously suitable for use in a fuelcell, especially a methanol fuel cell, since the way in which itfunctions is not impaired by the temperatures and ambient conditionsprevailing therein.

[0014] An advantageous embodiment of the inventive membrane according toclaim 2 contains as the cation-complexing agent at least one componentfrom the group of cryptands or cryptate-like compounds. The cycliccompounds on which these groups are based have a carbon or siliconskeleton.

[0015] By cryptand there is understood a macropolycyclic azapolyether,in which two bridgehead nitrogen atoms are joined by bridges containingone or more O atoms. By virtue of their three-dimensional structure, thecryptands are capable of forming extremely stable complexes known ascryptates with metal ions, especially alkali and alkaline earth ions.

[0016] Within the meaning of the invention, cryptand-like compounds areto be understood as compounds whose structure is analogous to that ofthe crown ethers but which are based on a silicon skeleton.

[0017] Further suitable components for a complexing agent of theinventive membrane are the crown ethers, which are defined as planarmacrocyclic polyethers, whose oxygen atoms are joined in particular byethylene bridges. In many cases benzene or cyclohexane rings are alsoattached by condensation. Simple monocyclic crown ethers can be readilysynthesized by reacting suitable polyethylene glycols with correspondingdichlorides in alkaline tetrahydrofuran solution.

[0018] Crown ethers have the property of forming coordination compoundsvery readily with metal ions, especially alkali and alkaline earth ions,and thus of transporting ions. The stability of the resulting crowncompounds is directly related to how well the cations fit into theligand cavity. Cryptands and the cryptate-like compounds have analogousproperties.

[0019] By virtue of the cation-complexing agents that it contains, theinventive membrane is capable, for example, of binding cations and thuspermitting the transport of anions, especially hydroxyl ions.

[0020] The inventive fuel cell according to claim 4 comprises an anode,a cathode and an inventive membrane disposed between the anode andcathode. The membrane is conducting in particular for anions. Theinventive membrane contains cation-complexing agents according toclaim 1. The use of such a membrane permits ionic transport of anionssuch as O²⁻ ions or hydroxyl ions through the membrane, but prevents thetransport of cations such as protons.

[0021] Complexing agents are compounds that are capable of formingcomplexes with metal ions, especially with alkali and alkaline earthions. Examples of advantageous cation-complexing agents that are capableof binding cations in complexes include crown ethers, cryptands orcryptate-like compounds, as claimed in claim 5 relating to the membraneof a fuel cell.

[0022] In the inventive method for operation of a fuel cell as set forthin claim 6, methanol is oxidized in an anodic reaction and oxygen isreduced in a cathodic reaction. Penetration of methanol into the cathodechamber is prevented by the inventive membrane according to claim 1. Inthe inventive method, only the oxide ions formed at the cathode or elsehydroxyl ions migrate in orderly fashion through the membrane to theanode, recombining with the fuel in the anode chamber to produce water.

1. A membrane containing cation-complexing agents.
 2. A membraneaccording to the preceding claim, characterized in that the complexingagent comprises at least one component from the group of cryptates orcryptate-like compounds based on carbon or silicon cyclic compounds. 3.A membrane according to claim 1, characterized in that the complexingagent comprises crown ethers.
 4. A fuel cell comprising an anode, acathode and a membrane disposed between anode and cathode, characterizedin that the membrane contains cation-complexing agents.
 5. A fuel cellcomprising an anode, a cathode and a membrane disposed between anode andcathode, characterized in that the membrane contains at least onecomponent from the group of crown ethers, cryptates or cryptate-likecompounds based on carbon or silicon cyclic compounds.
 6. A method foroperating a fuel cell, in which methanol is oxidized in an anodicreaction and oxygen is reduced in a cathodic reaction, characterized inthat anions pass through a membrane according to claims 1 to 3 from thecathode to the anode, and so penetration of methanol into the cathodechamber of the fuel cell is prevented.